For start-up of a cold fluidized bed combustion plant, it is known to provide special start-up combustion chambers by means of which the plant is heated up by the combustion gases. Water vapour in these gases may condense on any cold surfaces whose temperature is below that of the dew point. Any sulfur present in the fuel will form sulfur dioxide SO.sub.2. In the presence of water, sulfuric acid H.sub.2 SO.sub.4 is formed, so that any precipitated moisture becomes acidic and is highly corrosive. Dust which adheres to the moisture may become deposited on surfaces in gas cleaners and dust discharge systems provided and can cause clogging. To prevent the precipitation of moisture, a certain amount of heating may take place using dry gases from a special hot air boiler or by direct electric heating of those surfaces in the plant where moisture precipitation is likely to occur.